Process for inhibiting polymerization of sulfur trioxide in liquid state or oleum



Patented Dec. 27, 1949 PROCESS FOR INHIBITING POLYMERIZA- 'TION. OF SULFUR TRIOXIDE IN LIQUID STATE OR... OLEUM Harold G. McCann, Deer Park, N. Y., and Robert v I V. Townend, Arlington, N. J assignors to a I 7 Allied Chemical & Dye Corporation, New York,

N. Y., a corporation of New York No Drawing. Application September 24, 1948, Serial No. 51,128

10 Claims.

invention relates to the stabilization-of liquid sulfur trioxide (substantially pure S03) and high strength oleums.

At room temperature, liquid sulfur trioxide 2 tial vaporization of S03 or use of heating containers capable of withstanding relatively high pressures, as is the case when high temperatures are needed to melt the more refractory types of polymers. The invention also affords methods for polymerizes to various degrees. The three gen- 5 erally recognized S03 modifications melt at apaccomplishing the foregoing objectives. A furproximately 17 0., 32.5 C., and 62 0., respecther object of the invention is to provide stabiliztively. On standing at temperature below about ing agents which are efiective in small concen- 27 0.; in the presence of even small amounts trations' in inhibiting polymerization of S03, are of moisture, the form melting at about 17 C. "easily dispersed in liquid sulfur trioxide and high rapidly polymerizes to the higher melting forms, Strength oleums, and which, i d t e not the mass soon becomes solid, and meltsonly cha r d 'n app i y d p d y. under pressure, sometimes requiring temperatures A cord n o 0111 invention, it s been found up to 100 C. to effect complete liquefaction. that the incorporation of relatively small amounts High strength oleums of free so; strengths upcf the b ro compou d methyl b r or bcrcn Ward of about 80% tend to polymerize in similar trifiuoride-dimethyl etherate or mixtures of these fashion, degree of polymerization depending materials, with substantially anhydrous sulfur chiefly upon the S03 strength of the oleum and trioxide or oleums of S03 strength such that S03 temperatures at which the material is stored. In po y ers tend to form, effects the stabilization oleums of the lower portions of the S03 strength above described. The invention further comrange indicated, certain polymers form, the deprises the discovery of stabilizing agents, gree of polymerization increases on passage of those indicated, which posse s inherent p p ti e, and while the material may not solidify tiessuch that the stabilizing objectives of the completely, the ultimate mass may be a liquid invention are efiected by the mere addition of phase containing in suspension the various solid 5 the agents to the material to be stabilized, such polymers noted. properties affording the important advantage In many industrial processes, it is highly dethat no further processing of the mixed material sirable to use so; in its strongest obtainable form. d t il zin agent s n s ary. However, the foregoing characteristics of sulfur AS Stated b e, oleums O 3 tren of trioxide and high strength oleums create marked about and upward tendte po y t0 disadvantage relative to the storage, shipment greater or less extent. d p n stly up n and use of such materials. While, for example, S03 strength and time and temperature of storsulfur trioxide might be shipped in drums as It has n nd t at by d pers n ou equipped with heating coils, there is involved the ubl a i r n p u ds i liqu d su first problem of supplying a relatively expensive fur trioxide or in high S ngth oleums 0f the form of container. Moreover, since heat transp n d, p lym riz i is inhibited 0r fer thru a solid material is poor, the solidifi d minimized. Since anhydrous sulfur trioxide tends sulfur trioxide in the container can be melted'by polymerize to a lesser degree than does Sulfur the user only with considerable difficulty. In trioxide Containing Water e., chemically comaddition to this disadvantage, after relatively long billed in the form of H2504), and Since, from periods of standing, a large proportion of the sula commercial Point Of View. t is d t eous fur trioxide wi11 have polymerized t t g .to manufacture, store and ship sulfur trioxide elt ng form in which case complete meltin can as such (in substantially anhydrous form) or as only be eifected by heating at high temperature an oleum containing not less than 99.5% free under considerable pressure. To a lesser extent Particular embodiments of 9 invention th same disadvantages apply to high strength are directed to stabilization of these high S03 oleums. content materials.

This invention aims to provide stabilized forms When boron trifillcride-dimethyl ate is of liquid sulfur trioxide or high strength oleums p y d a inhibitor r u fur rioxide in the which either do not polymerize to any substantial manner herein described, substantially completely degree and remain liquid at room temperature, stabilized materials may be obtained. Accordor in which, if polymerization does take place, h lht ethelate iS the p e ed P y e a on the polymers formed are readily meltable at-modinhibitor. When methyl borate is utilized as staerate, temperatures of about 40 C. and below, bilizing agent, amounts of polymer formed may e. temperatures which do not require 'substanbe somewhat greater than in the case of the sired and the amount of water present in the In general, the larger theamountcf:

material. agent added, the greater is the stabilizing effect thereof; and, considering anhydrous sulfur trioxide (containing substantially-nowateryas a starting point, the greater the-amount of water, the greater is the tendency for S03. polymers to form, and the greater will be the amountofstabilizer required to effect a given degree of stabilization. For substantially anhydrous sulfur trioxide adequate stabilization may be obtained particular-theory, --we believe that the organic stabilizingagents of; this invention dissolve in fur trioxide has sufficiently low vapor pressure. Similarly, liquid sulfur trioxide may be obtained from the dry mist-free, say 10%, S03 exiting a catalytic S02. oxidation, operation, by compressing -theI- gasto.ab out 160 170.;lbs. per square inch, and cooling the compressed'gas to -25 0., un-

,der which conditions about 70% of the S03 may be liquefied and recovered as substantially 100% liquidsulfur trioxide.. Whi-le we do not intend to limit ourselves to any ,theoleumorsulfur trioxide, thereby forming solutions .ofporganicaboron compound in the material stabilized. This solubility of our organic agents accounts for the ease of incorporation and by employing either treating agent orv mixtures,

thereof in amount equivalent to about 0.01% by height of boronor greater. By using etherate inhibitor in amounts-equivalent to at least 0.03% by weigh-tof boron, anhydrous sulfur trioxide may-be rendered substantially completely stable, icennateri-al may be stored indefinitely with no substantial polymerization. I

-;;'W-hen stabilizing oleums of S03 strengthbelow 10.9%: but equal to or greater than 99.5 since the combined water present enhancesthe tendency of polymer. :formation, inhibitor equivalent to 0.03 by weight of boron or greater are-, preferredl. .7 Such stabilized materials do; not polymerize to an appreciable extent (i. e., less :tl'xam;1:-e21%)-and remain substantailly in liquid iormf when "broughtto room temperature even though: previously stored for several weeks at temperature of zero degrees C. or less. As indicated above, anysuch polymer formed is readfly Ieconver-ted to monomeric S03 by warm-ingto 4.0?150. or'below. Hence, products of this type are completely acceptable in nearly all commercial applications. When stabilizing sulfur trioxideor oleums ofhigher S03 strengths, it is preferable "to employ substantially anhydrous agents inorder tomai'ntainElow water content of the end product. tilt is usually desirable to avoid dilutioncflf the ztrioxide and to maintain SOs strength-at the maximum value possible. Therefore: it is preferred. tov utilize treating material in amount equivalent to not more than 0.6% by weight of boron, the latter: quantity being generally-;.ap- 'preciablymore than adequate to effect stabilizationzof sulfur trioxide .or high strength oleums when the sought-for product is such that if the product contains polymers such polymers are readily.meltable at temperature below about 40 13'. ..Moreover, we find that over-all results :are notssignifican'tly better when the stabilizing agents ofcthe invention are used in quantities much in excess of the equivalent to 0.6% by weight of boron... 1:; The preferredzproducts of the invention. 1. e., iltleyinhibited sulfur trioxide. or oleums of S02 strength not less than 99.5% which do not polymerize to any appreciable extent preferably should begkept sealed oif from contact with air. since a small amount of moisture causes such products to lose some stability and to form these low meltins polymers.

The sulfur trioxide or oleum to be treated-may be obtained from any suitable source. -For example ,substantially 100 S03 may be obtainedby distilling say 30% oleum under conditions to avoid vaporization of HzSOt, and condensing the-S03 at temperature, say 20-30 C., at which liquid sulconcentrations dispersion of inhibitor in the oleum or sulfur trioxide, and affords the marked advantage of the present invention in that no further processing of the mixed S03 material and stabilizing agent 5. e u red-.11 ix; m 1

{lihetreating agent used maybe added directly to the liquid to be stabilizedand dissolyedby agitation or in the case of stabilizing substantially 1.00 sulfur trioxide. the vaporous S03 convenientlymay be co ndensedin a cl'osed vessel in which the treating material to be used has been prew r d- L, The iollowingexamples illustrate the practice of our invention, the parts being by weight;

E zcqntple ;1.--B,oron trifiuoride dimethyl etherateinzamount equivalentyto about 0.05% by weight of boron was added to anhydrous sulfur trioxide and the; mixture mildly agitated at. substantially room temperature. The resulting mixture .was stored for one month at aboutm'inusfl to minus 6016., and at the endpfthis time. was brought to ;room temperature. The sulfur trioxide :was completely unpolymerized, i. e. contained no unmelted solid. I Example Z Methyl borate .in amount equivalent to 0.06% by weight of boron was added to anhydrous sulfur trioxide and the mixture mildly agitated at about room temperature. The retin mixture was then stored at .about0" C. for two weeks, after which time :it was warmed to room -tempera;ture. No unmelted solid remaine dlin the material. Storage at 0 C. was continued for a total of two months. At the end of this time the material melted'to a liquid containing about 1% solids which could readily be transferred from the container by pumping. These solids were melted '(i. e., depoly-meriz'ed) 'by maintaining the material at about 40 C. for a short periodof time. I Egcample 3.Boron trifiuoride-dimethyletherate in amount, equivalent to about 0.10% by weight of boron was mixedwith sulfur trioxi'de. The water,ycontent. (chemically combined) of the resulting mixture was. 0.035% (approximately 99.8 $03) based on H2504 plus S03. This boron treated oleum was storedatzero to minus 60 for one month and then warmed 'to'room temperature, About 1% solid material remained un nelted. g Ecnmple 4.-Methyl borate in amount equivalent to about 0.02% by weight of boron was added ;to anhydrous sulfurtrioxide and thei'mixq-ture mildly agitated at about .room temperature. The resulting mixture was then stored at about .Q",..C.1 fortwo weeks, after which time it was warmedto room temperature. A trace of unmeltedsolid, remained' in the material" Storage at :0 C. was continued ;for.-.a total of two months. At the end-of this time the material melted to a liquid containing less than 5% solids which could readily be transferred from the container by pumping. These solids were melted (i. e., depolymerized) by maintaining the material at about 40 C. for a short period of time.

We claim:

1. The method of stabilizing material of the group consisting of sulfur trioxide and oleum of S03 strength such that S03 polymers tend to form, which method comprises incorporating with said material organic boron compound, of the group consisting of methyl borate, boron trifluoride-dimethyl etherate and mixtures thereof. in amount sufficient to stabilize said material.

2. The method of stabilizing material of the group consisting of sulfur trioxide and oleum of S03 strength such that $03 polymers tend to form, which method comprises incorporating with said material organic boron compound, of the group consisting of methyl borate, boron trifiuoride-dimethyl etherate and mixtures thereof, in amount sufiicient to stabilize said material but in amount equivalent to not more than 0.6% by weight of boron.

3. The method of stabilizing material of the group consisting of sulfur trioxide and oleum of S03 strength not less than 99.5% which method comprises incorporating with said material organic boron compound, of the group consisting of methyl borate, boron trifiuoride-dimethyl etherate and mixtures thereof, in amount sufficient to stabilize said material.

4. The method of stabilizing material of the group consisting of sulfur trioxide and oleum of S03 strength not less than 99.5%, which method comprises incorporating with said material organic boron compound, of the group consisting of methyl borate, boron trifiuoride-dimethyl etherate and mixtures thereof, in amount equivalent to 0.03 to 0.6% by weight of boron.

5. The method of stabilizing material of the group consisting of sulfur trioxide and oleum of S03 strength not less than 99.5%, which method comprises incorporating with said material subv 6 stantially anhydrous boron trifiuoride-dimethyl etherate in amount sufficient to stabilize said material.

6. The method of stabilizing substantially anhydrous sulfur trioxide which comprises incorporating with said sulfur trioxide substantially anhydrous boron trifluoride-dimethyl etherate in amount equivalent to 0.01 to 0.6% by weight of boron.

7. A stable composition resistant to polymerization comprising material of the group consisting of sulfur trioxide and oleum of S03 strength such that S03 polymers tend to form, and containing a relatively small amount of boron derived from an incorporated organic substance of the group consisting of methyl borate, boron trifluoride-dimethyl etherate and mixtures thereof.

8. The composition comprising material of the group consisting of sulfur trioxide and oleum of S03 strength not less than 99.5% and containing boron compound derived from incorporated organic substance of the group consisting of anhydrous methyl borate, anhydrous boron trifiuoride-dimethyl etherate and mixture thereof, the amount of said compound being equivalent to 0.03 to 0.6% by weight of boron.

9. A composition comprising material of the group consisting of sulfur trioxide and oleum of S03 strength not less than 99.5%, and containing boron compound derived from incorporated anhydrous boron trifluoride-dimethyl etherate, the amount of said compound being suflicient to stabilize said material.

10. A composition comprising substantially anhydrous sulfur trioxide containing a boron compound derived from incorporated anhydrous boron trifluoride-dimethyl etherate, the amount of said compound being equivalent to 0.01 to 0.6% by weight of boron.

HAROLD G. MCCANN. ROBERT V. TOWNEND.

No references cited. 

